Platelet-like colored pigments and process for producing the same

ABSTRACT

Platelet-shaped colored pigments containing titanium dioxide, one or more suboxides of titanium and an oxide or oxides of one or more metals other than titanium or non-metals and a process for the preparation thereof is disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This is a 371 of PCT/EP93/00617, filed Mar. 16, 1993.

The present invention relates to a platelet-shaped colored pigment comprising titanium dioxide, one or more suboxides of titanium, and an oxide or oxides of one or more different metals and/or non metals.

BACKGROUND OF THE INVENTION

There is a continuously increasing demand for pigments with intense pearlescent color and/or metallic luster in the field of paints, coatings, inks, plastics, cosmetics and especially for exterior coatings. Therefore, the development in the field of pigments aims at lustrous and hiding pigments which can be new effect pigments or replace the metallic pigments such as aluminum flakes with their known disadvantages. It is known that coatings of TiO₂ -layers on platelet-shaped substrates produce the so-called pearlescent effect. For this kind of pigments it is possible to control the interference color by changing the thickness of the TiO₂ -layer. It is known from DE 3433657, U.S. Pat. No. 4,623,396 and EP 0332071 to use suboxides, e.g. suboxides of titanium, for the coating of platelet-shaped substrates whereby for the reduction of the TiO₂ ammonia gas is used.

However, the reduction of solid particles by a reducing gas is not suitable for an industrial production, because the color of the product as a function of the degree of reduction is difficult to control. It is dependent on the size of the particles, the temperature, the gas flow and the time of treatment. Furthermore special facilities for dangerous or toxic gas are necessary. It is also known that such suboxides provide functional properties such as electric conductivity.

JP-A-1-158077 discloses a pigment consisting of a titanium oxide layer containing a dark color region on a mica core and a color tone adjusting layer as covering layer comprising at least one of silicon oxide, aluminum oxide and zinc oxide or a composite oxide thereof. The dark color region is composed of a dark color metal oxide, such as titanium oxide or iron oxide of lower order, of titanium nitride, titanium oxide or carbon black.

The pigment is manufactured in a first step by heating and reducing titanium dioxide-coated mica flakes at a temperature of from 500° C. to 1000° C. in a reducing gas; or by blending titanium dioxide-coated mica flakes with titanium metal and heating the resulting blend in a vacuum at a temperature of from 500° to 1000° C. for a long time of more than 6 hours. The covering layer is deposited in a second step in an aqueous medium by precipitating of silicon dioxide hydrate on the first layer containing the titanium suboxides and drying or firing the pigment. This additional coating improves heat stability of the suboxide containing layer. However, the functional properties such as electric conductivity of the first layer is shielded by this second layer. The pigment has the disadvantage that it is manufactured in a complicated two-step process which raises the price. The reduction in vacuum requires expensive facilities and complicated operation and high temperature and longer reaction time causes low production efficiency. Furthermore the used titanium metal is expensive.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide platelet-shaped colored pigments having improved heat stability without losing the functional properties of colored layers which are producible in a simple one-step process.

This object is solved according to the present invention by a platelet-shaped colored pigment on the basis of a platelet-shaped substrate which is covered by a coating layer containing TiO₂, one or more suboxides of titanium and an oxide or oxides of one or more different metals and/or non metals. The concentration of said titanium oxides in the coating layer is maximum in the proximity of the substrate surface and gradually decreases towards the pigment surface and the concentration of said oxide or oxides of one or more different metals and/or non metals is maximum at the pigment surface and gradually decreases towards the substrate surface whereby mixed phases of these oxides exist inside the coating layer.

The different elements are preferably selected from alkali metals, alkaline earth metals, B, Al, Si, Zn or Fe. Furthermore all other elements with sufficient reactivity can be used.

The amount of the one or more different metals and/or non metals is from 1 to 50% by weight, preferably from 2 to 30% by weight, relative to the amount of titanium in the pigment.

Furthermore this object is solved by a process for producing platelet-shaped colored pigments by mixing a platelet-shaped substrate coated with TiO₂ with at least one solid reducing agent, preferably an alkaline earth metal B, Al, Si, Zn, Fe, LiH, CaH₂, Al₄ C₃, Mg₂ Si, MgSi₂, Ca₂ Si or CaSi₂ and heating the mixture in a non-oxidative gas atmosphere at a temperature of more than 600° C. for more than 10 minutes.

As starting materials platelet-shaped substrates like mica, kaolin or glass platelets which optionally may be coated with one or more metal oxide layers, or metal or metal oxide platelets coated with TiO₂ are used. The particle sizes of the substrates are 1 to 500 μm in diameter and 0.1 to 1 μm in thickness. The materials are coated with TiO₂ according to processes well-known in the art and described for example in U.S. Pat. No. 3,553,001.

The TiO₂ -layer has a thickness in the range of from 10 to 1000 nm, preferably from 40 to 500 nm. The reduction reaction takes place in a non-oxidative gas atmosphere such as N₂, Ar, He, CO₂, C_(x) H_(y), H₂, NH₃. N₂ or Ar are preferred. In case of N₂ or NH₃, TiN or TiON may be formed in addition to TiO_(2-x). It forms mixed solid suboxides or oxide bronzes on the platelet-shaped substrates as soft solid powders at temperatures of more than 600° C., preferably in the range from 700° to 1100° C. for more than 10 minutes, preferably for 15 minutes to 60 minutes. Some examples will illustrate the reactions: ##STR1##

As reducing agent fine powdered elements like for example Al, Si, Mg, Ca and B or combinations thereof are particularly preferred. Furthermore alloys of metals or metal borides, carbides and silicides can be used.

Well-known reducing agents like the alkali metals may be used in liquid or gas phase.

Other reducing agents are hydrides like LiH or CaH₂.

In addition, combinations of these reducing agents with each other are possible.

The TiO₂ -pigment and the reducing agent are mixed in a ratio from 200:1 to 5:1 preferably from 100:1 to 10:1. The obtained pigments exhibit strong pearlescent color and/or metallic luster.

The color effect of the pigments can be controlled by changing

the particle size of the platelet-shaped substrate (smaller particles lead to a soft silky luster, larger particles lead to a glittering luster),

the thickness of the Ti-oxides layer (interference color),

the kind of the solid reducing agent (low reduction potential yields grey to blueish black, high reduction potential yields black to yellowish black) and

the amount of the solid reducing agent.

The reduction reaction is accelerated in the presence of a halide, preferably a chloride as pointed out in Table 1. LiCl, NaCl, KCl, MgCl₂, CaCl₂, CuCl₂, CrCl₃, MnCl₂, FeCl₂, FeCl₃, COCl₂, NiCl₂ or CeCl₃ are preferred. The reaction temperature can be reduced in the presence of chloride by 150° to 300° C. In example I the calcining temperature has to be 1000° C. without the admixture of chloride in order to reach a reaction. It is reduced to 840° C. by admixing of CaCl₂.

The amounts of the halide can vary from 0.1 to 40%, preferably from 0.5 to 10% relative to the platelet-shaped substrate coated with TiO₂.

The products show a depth profile regarding the distribution of titanium and the one or more different metals and/or non metals inside the coating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a depth profile of Iriodin 120, a pearlescent TiO₂ -pigment manufactured by E. MERCK. The concentration of titanium on the pigment surface is 22 atomic % contrary to the silicon concentration which is only about 5 atomic %.

FIG. 2 shows a depth profile of the same pigment after the reduction with metallic silicon according to example 1. The titanium concentration is decreased to 2 atomic % and the silicon concentration is increased to more than 28 atomic %. During the reduction reaction metallic silicon penetrates into the TiO₂ -layer and is oxidized to Si₂. The silicon concentration decreases continously in the direction to the substrate and is about 9% in a depth of 30 nm.

FIG. 3 shows the distribution of titanium and silicon in the pigment described in JP-A-1-158 077 beginning from the surface of the coating layer in direction to the substrate. The known pigment has been manufactured by a two-step process as described in the comparative examples 1 and 2. The two pigments show a very different structure. The known pigment has distinct layers. The top layer precipitated from an aqueous medium containing approximately 100% of silicon dioxide is more than 3 nm thick. In contrary, the pigment according to the present invention does not possess any distinct layers. The concentration of the components in the coating layer is gradually changing with the depth of this layer.

A pigment which is used in an organic solvent system such as paint or ink is often required to have a certain level of conductivity in order to avoid the danger of explosion by electrostatic spark. Table 2 shows the temperature resistance and the electroconductivity of the known pigment after the first process step (reduction of TiO₂ -layer with Ti metal; PM 21-2) and after the second process step (precipitation of Si₂ ; PM 21-3) and of the pigment according to the present invention (PM 21-1). The pigment according to the present invention has a significantly better electroconductivity than the pigment produced by the two-step process. Additional coating with SiO₂ (PM 21-3) reduces the conductivity.

Furthermore the pigments according to the present invention show a better application behavior. That means better results regarding the humidity test in car paint application and regarding the photoactivity test in out-door application. Photoactivity test results are shown in Table 3. The pigment according to the invention has been compared with known pigments.

If desired the pigment can be provided with high magneticity as shown in Table 4.

The invention will now be described more specifically with reference to examples thereof, as well as comparative examples, which are not intended to limit the scope of the invention.

EXAMPLE 1

15 g of a TiO₂ -coated mica powder, particle size distribution 5-25 μm, exhibiting white mass tone with white reflection color (Iriodin® 120; produced by E. Merck) and 1.5 g of Si powder, particle size <150 m (from E. Merck), was mixed thoroughly in a 500 ml plastic bottle.

The mixture was put into a quartz boat.

The boat was placed inside a quartz tube (inner diameter 5 cm, length 100 cm). Both ends of this tube were equipped with gas pipes, one for introducing gas and the other for exhausting gas.

Then blowing of N₂ gas into this tube was started at room temperature with the flow rate of 0.25 1/min.

After 15 min., the tube was placed into an oven which was kept at 1000° C., and maintained for 30 minutes.

The tube was put out of the oven and let cool down. Until the tube was cooled down, the blowing of N₂ gas was maintained. The calcined powder was washed with water and dried at 120° C. over night, and then the powder was passed through a 40 μm sieve.

To evaluate the color characteristics of the pigment produced according to the invention, 0.9 g of each pigment sample is worked in 53.6 g of an acrylic modified nitrocellulose lacquer. After homogenizing with a propeller mixer at 1000 rpm for 10 min the dispersion is allowed to stand for one hour to remove air bubbles. Then the pigmented lacquer is coated with an applicator (500 μm wet film thickness) on a block and white card.

The visual evaluation of the color is expressed as masstone and interference color. The masstone color is the observation out of luster (45°/0°) and the interference color is the observation at reflection angle (22.5°/22.5°).

The CIE Lab color coordinates are measured with Hunter Lab Color Meter model D-25 under the conditions of 45°/0° with white background and 22.5°/22.5° with black background. The obtained powder exhibits blue black masstone with blue interference color (coloristics see also Table 4).

By ESCA measurement with combination of Ar sputtering technique, depth distribution of Si atoms in the coated layer of this pigment was determined.

As shown above in FIG. 2, Si atoms penetrate into Ti-oxide layer more than 30 nm and the distribution of Si atom shows gradation of concentration along the depth. X-ray diffraction of this powder indicates the presence of γTi₃ O₅ and SiO₂ (quartz).

EXAMPLE 2

100 g of a TiO₂ -coated mica powder, particle size distribution 5-25 μm, exhibiting reddish white mass tone with green interference reflection color (Iriodin®231; produced by E. Merck), 3 g of the same Si powder as in Example 1 and 1 g of CaCl₂ were mixed throughly in a 3 l plastic bottle. This mixture was calcined at 800° C. for 30 minutes under N₂ gas flow of 3.0 l/min. with the same manner as in Example 1. The calcined powder was washed with water and dried at 120° C. overnight, and then the powder was passed through a 40 μm sieve.

The obtained powder exhibits bluegreen black masstone with bluegreen interference color. This sample showed 7.5 kΩ. cm of powder conductivity which is enough to avoid electrostatic spark (see Table 2).

Comparative Example 1

This comparative example demonstrates the reduction with metallic titanium according to JP-A-1-158 077 as the first step of the two-step process.

100 g of the same TiO₂ -coated mica as in Example 2 and 5 g of Ti powder (particle size 150 gm, from E. Merck) were mixed throughly in a 3 l plastic bottle.

This mixture was calcined at 1000° C. for 30 minutes under N₂ gas flow of 0.25 l/min. with the Same manner as in Example 1. The calcined powder was washed with water and dried at 120° C. overnight, and then the powder was passed through a 40 μm sieve.

The obtained pigment exhibits bluegreen black mass tone with blue green interference color.

The sample showed a powder conductivity of 31.5 MΩ.cm. Consequently, the pigment according to the present invention has a significantly better electroconductivity than the comparison pigment.

Comparative Example 2

As the second step of the two-step process described in JP-A-1-158 007 silicon oxide hydrate was deposited on the surface of the pigment obtained according to Comparative Example 1.

50 g of the pigment was dispersed into 500 ml of deionized water with stirring. During the reaction the stirring was maintained. The dispersion was heated up to 90° C. and the pH was adjusted by adding 5 wt % NaOH aq. solution. To this dispersion 75 ml of 10 wt % Na₂ SiO₃ aq. solution was added at the rate of 2 ml/min. While keeping the pH at 9 by simultaneous addition of 1N HCl aq. solution. 62 ml of 1N HCl aq. solution was needed.

The temperature and stirring were maintained for an additional hour and then the mixture was cooled. The solid was filtered, washed with water and dried at 200° C. overnight. The obtained pigment exhibits bluegreen black mass tone with bluegreen interference color.

ESCA analysis with combination of Ar sputtering technique indicates that a more than 3 nm thick silicon oxide layer, covers the titanium oxide layer (see FIG. 3).

The sample shows a powder conductivity of >125 MΩcm. This value makes clear that the conductivity of the first layer is shielded by the silicon oxide layer.

EXAMPLE 3-14

Mixtures of a TiO₂ -coated mica pigment and a reducing agent and in some cases a chloride were prepared in the same manner as in Example 1. The ratio of the reactants is listed in Table 4. The mixtures were calcined under the conditions described in Table 4.

The calcined powders were washed with water, dried at 120° C. overnight and then passed through a 40 μm sieve.

The appearance colors and measured color values are listed in Table 4.

All of these Examples exhibit grey to dark color indicating the formation of suboxides by the reaction.

EXAMPLES 15-32

These Examples demonstrate the influence of halides which accelerate the reduction reaction and which make it possible to lower the reduction reaction temperature.

The samples were prepared in the same manner as in Example 1. The mixing ratios and the calcination conditions are listed in Table 4.

The color appearance and the measured values are described in Table 4.

The colors of the pigments in the Examples 16-19 are darker than the color of the pigment in Example 15. In this Example no halide was added. The darker color indicates the reduction reaction was accelerated.

The colors of the pigments in the Examples 20-28 are dark indicating a reduction reaction took place, but without a halide the reduction reaction does not take place at such low temperatures.

The Examples 29 to 32 show that with decreasing amount of the halide in the mixture the color of the pigment becomes lighter.

EXAMPLE 33

This Example demonstrates the production of a dark colored conductive pigment.

15 g of TiO₂ -coated mica pigment (the same as in Example 1) and 3 g of an Al powder (particle size <250 μm, from E. Merck) was mixed in the same manner as in Example 1 and calcined at 1000° C. for 30 minutes under N₂ gas flow of 0.25 l/min.

The calcined material was washed with water, dried at 120° C. over night and passed through a 40 μm sieve.

The obtained pigment exhibits yellowish black mass tone with brown reflection color.

The X-ray diffraction of this pigment indicates the presence of Ti₂ O₃, TiN (Osbornite syn) and A₂ O₃.

The electroconductivity of this pigment was determined as 20 Ω.cm.

EXAMPLE 34

This Example demonstrates the production of a pigment with magnetic properties. 15 g of the same TiO₂ -coated mica pigment as in Example 2, 1.35 g of the same Si powder as in Example 1 and 0.15 g of CaCl₂ were mixed in the same manner as in Example 1. The mixture was calcined at 1000° C. for 30 minutes under N₂ gas flow of 1.5 l/min.

The calcined material was washed with water, dried at 120° C. overnight and passed through a 40 μm sieve. The obtained pigment exhibits black mass tone with brown reflection color and magnetic properties.

                                      TABLE 1                                      __________________________________________________________________________     Example                                                                              TiO.sub.2 coated                                                                      Reduction                                                                            Mixing ratio                                                                          Gas   Calcine                                                                             Color masstone/                           No.   mica pig.                                                                             agent pig./red./chl.                                                                        flow rate                                                                            temp.                                                                               interference                              __________________________________________________________________________     I     Ir. 123                                                                               Si/CaCl.sub.2                                                                        100/4/4                                                                               N.sub.2                                                                              840° C.                                                                      blue black/                               PM 11                     0.25 l/min blue                                      comparison                                                                           Ir. 123                                                                               Si    100/4  N.sub.2                                                                              840° C.                                                                      no reaction                                                         0.25 l/min (white/white)                             II    Ir. 123                                                                               Al/KCl                                                                               100/1/0.25                                                                            N.sub.2                                                                              800° C.                                                                      grey/white                                PM 13-2                   0.25 l/min                                           comparison                                                                           Ir. 123                                                                               Al    100/1  N.sub.2                                                                              800° C.                                                                      no reaction                                                         0.25 l/min                                           III   Ir. 123                                                                               B/KCl 100/1/0.25                                                                            N.sub.2                                                                              600° C.                                                                      light grey/                               PM 12-4                   0.25 l/min white                                     comparison                                                                           Ir. 123                                                                               B     100/1  N.sub.2                                                                              600° C.                                                                      no reaction                                                         0.25 l/min                                           __________________________________________________________________________      Ir. means Iriodin ®, a trade name for interference pigments,               manufactured by E. MERCK.                                                

                                      TABLE 2                                      __________________________________________________________________________                                Process                                                    TiO.sub.2 coated                                                                      Reduction                                                                            Mixing ratio                                                                          Gas   Calcine                                                                             Electro-                                 Sample mica pigment                                                                          agent pig./red.                                                                             flow rate                                                                            temp.                                                                               conductivity*                            __________________________________________________________________________     Example 2                                                                             Ir. 231                                                                               Si/CaCl.sub.2                                                                        100/3/1                                                                               N.sub.2                                                                               800° C.                                                                     7.5 kΩ · cm               (PM 21-1)                  3 l/min.                                            Comparative                                                                           Ir. 231                                                                               Ti    100/5  N.sub.2                                                                              1000° C.                                                                     31.5                                                                               MΩ · cm               Example 1                  0.25 l/min.                                         (PM 21-2)                                                                      Comparative                                                                           PM 21-2 is coated with SiO.sub.2.nH.sub.2 O                                                                   >125                                                                               MΩ · cm               Example 2                                                                             Na.sub.2 SiO.sub.3 + HCl → SiO.sub.2.nH.sub.2 O                  (PM 21-3)                                                                      __________________________________________________________________________      *Specific restistance of pigment powder, resistance of pressed powder          between two metal pistons in a tube with 10 kg/3 cm.sup.2 was measured an      calculated according to following formula:                                     ##STR2##                                                                       (R.sub.sp : specific resistance [Ω · cm]); R: measured          resistance [Ω]; A: crosssectional area of a tube [cm.sup.2 ]; d:         thickness of powder layer [cm])                                          

                                      TABLE 3                                      __________________________________________________________________________         TiO.sub.2 coated                                                                      Reduction                                                                            Mixing ratio                                                                          Gas   Calcine                                                                             Color masstone/                                                                         Humidity.sup.1)                                                                      Photoactivity.sup.2)         Sample                                                                             mica pig.                                                                             agent pig./red./chl.                                                                        flow rate                                                                            temp.                                                                               interference                                                                            16 h/66° C.                                                                   UV lamp, 24                  __________________________________________________________________________                                                       h                            1   Ir. 103                                                                               Mg    100/3.8                                                                               Ar    800° C.                                                                      dark grey/                                                                              A     B                                                    0.25 l/min silver                                      2   Ir. 103                                                                               Si/CaCl.sub.2                                                                        100/2/0.5                                                                             N.sub.2                                                                              900° C.                                                                      grey/silver                                                                             B     B                                                    3 l/min.                                               Ir. 103                                                                            --     --    --     --    --   white/white                                                                             C     C                            3   Ir. 123                                                                               Si/B/KCl                                                                             100/4/1/0.5                                                                           N.sub.2                                                                              700° C.                                                                      dark grey/                                                                              B     B                                                    3 l/min.   silver                                      Ir. 123                                                                            --     --    --     --    --   white/white                                                                             C     C                            __________________________________________________________________________      .sup.1) Test system 1coat acrylicmelamine thermosetting paint. 4 wt %          pigment content based on total paint.                                          .sup.2) 1 phr pigment incorporated in Pb compound containing PVC plate.        rank                                                                           A: less degradation                                                            B: fair                                                                        C: much degradation                                                            Ir. means Iriodin ®, a trade name for interference pigments,               manufactured by E. MERCK.                                                

                                      TABLE 4                                      __________________________________________________________________________     Powder mixture          Calc. conditions                                                  Reduction                                                                            Mixing ratio Calcine                                          Example                                                                              TiO.sub.2                                                                           agent/                                                                               pig./red./                                                                            Gas   temp.                                                                               Color masstone/                             No.   mica pig.                                                                           halide                                                                               halide flow rate                                                                            (30 min.)                                                                           interference                                __________________________________________________________________________     1     Ir. 120                                                                             Si    100/10 N.sub.2,                                                                             1000° C.                                                                     blue black/                                 (Be 462)                0.25 l/min.                                                                               blue                                        2     Ir. 231                                                                             Si/CaCl.sub.2                                                                        100/3/1                                                                               N.sub.2                                                                               800° C.                                                                     bluegreen                                   (PM 21-1)               3.0 l/min. black/bluegreen                             3     Ir. 120                                                                             Al    100/10 N.sub.2,                                                                             1000° C.                                                                     black/                                      (Be 454)                0.25 l/min.                                                                               reddish white                               4     Ir. 120                                                                             Mg    100/10 Ar,    800° C.                                                                     black/                                      (Be 564)                0.25 l/min.                                                                               silver                                      5     Ir. 221                                                                             B     100/1  N.sub.2,                                                                              900° C.                                                                     blue black/                                 (Hg 34)                 0.25 l/min.                                                                               lilac                                       6     Ir. 120                                                                             Al.sub.4 C.sub.3                                                                     100/5  N.sub.2,                                                                             1000° C.                                                                     blueish                                     (Hg 1)                  0.25 l/min.                                                                               grey/silver                                 7     Ir. 120                                                                             Mg.sub.2 Si                                                                          100/5  N.sub.2,                                                                             1000° C.                                                                     blueish                                     (Hg 8)                  0.25 l/min.                                                                               grey/silver                                 8     Ir. 120                                                                             MgSi.sub.2                                                                           100/5  N.sub.2,                                                                             1000° C.                                                                     blueish                                     (Hg 14)                 0.25 l/min.                                                                               black/lilac                                 9     Ir. 120                                                                             Ca.sub.2 Si                                                                          100/5  N.sub.2,                                                                             1000° C.                                                                     grey/silver                                 (Hg 15)                 0.25 l/min.                                            __________________________________________________________________________     Color meter values                                                                   45°/0°                                                                       22.5°/22.5°                                                                 Electro-                                          Example                                                                              white background                                                                           black background                                                                          conductivity                                      No.   L   a   b   L  a   b   tivity  Magnetivity                               __________________________________________________________________________     1     13.2                                                                               -1.3                                                                               -7.4                                                                               38.8                                                                              +4.6                                                                               -15.5                                                                              3.0 kΩcm                                                                         None                                      (Be 462)                                                                       2     --  --  --  -- --  --  7.5 kΩcm                                                                         --                                        (PM 21-1)                                                                      3     13.6                                                                               -0.5                                                                               -0.8                                                                               37.7                                                                              +3.8                                                                               -0.9                                                                               -1.3                                                                               kΩcm                                                                         Little                                    (Be 454)                                                                       4     18.8                                                                               -0.6                                                                               -1.0                                                                               41.6                                                                              +2.4                                                                               -2.2                                                                               1.8 MΩcm                                                                         Little                                    (Be 564)                                                                       5     34.9                                                                               -2.0                                                                               -5.4                                                                               36.8                                                                              +20.3                                                                              -39.5                                                                              451 kΩcm                                                                         Little                                    (Hg 34)                                                                        6     27.1                                                                               -2.9                                                                               -7.9                                                                               59.2                                                                              +0.5                                                                               -13.1                                                                              30  kΩcm                                                                         Little                                    (Hg 1)                                                                         7     21.1                                                                               -1.7                                                                               -4.5                                                                               53.4                                                                              +2.1                                                                               -8.7                                                                               24.3                                                                               kΩcm                                                                         Much                                      (Hg 8)                                                                         8     12.3                                                                               -0.3                                                                               -3.6                                                                               37.6                                                                              +3.9                                                                               -8.5                                                                               12.2                                                                               kΩcm                                                                         Much                                      (Hg 14)                                                                        9     42.5                                                                               -2.7                                                                               -3.3                                                                               72.7                                                                              +1.5                                                                               -6.7                                                                               155 MΩcm                                                                         Much                                      (Hg 15)                                                                        __________________________________________________________________________     Powder mixture          Calc. conditions                                                  Reduction                                                                            Mixing ratio Calcine                                          Example                                                                              TiO.sub.2                                                                           agent/                                                                               pig./red./                                                                            Gas   temp.                                                                               Color masstone/                             No.   mica pig.                                                                           halide                                                                               halide flow rate                                                                            (30 min.)                                                                           interference                                __________________________________________________________________________     10    Ir. 120                                                                             CaSi.sub.2                                                                           100/5  N.sub.2,                                                                             1000° C.                                                                     blueish                                     (Hg 7)                  0.25 l/min.                                                                               grey/silver                                 11    Ir. 120                                                                             LiH   100/5  N.sub.2,                                                                             1000° C.                                                                     yellowish                                   (Hg 10)                 0.25 l/min.                                                                               grey/silver                                 12    Ir. 123                                                                             Fe/CaCl.sub.2                                                                        100/5/5                                                                               N.sub.2,                                                                              800° C.                                                                     yellowish                                   (PM 30-1)               3.0 l/min. grey/silver                                 13    Ir. 123                                                                             Zn/CaCl.sub.2                                                                        100/5/5                                                                               N.sub.2,                                                                              600° C.                                                                     grey/silver                                 (PM 30-5)               3.0 l/min.                                             14    Ir. 123                                                                             Si/B/KCl                                                                             100/4/1/0.5                                                                           N.sub.2,                                                                              650° C.                                                                     grey/silver                                 (MK 24)                 0.25 l/min.                                            15    Ir. 221                                                                             Si    100/10 N.sub.2,                                                                             1000° C.                                                                     yellowish                                   (Be 523)                4.0 l/min. grey/blue                                   16    Ir. 221                                                                             Si/LiCl                                                                              100/9/1                                                                               N.sub.2,                                                                             1000° C.                                                                     darkblue/                                   (Hg 29)                 1.5 1/min. darklilac                                   17    Ir. 221                                                                             Si/NaCl                                                                              100/9/1                                                                               N.sub.2,                                                                             1000° C.                                                                     darkblue/                                   (Hg 18)                 1.5 1/min. lilac                                       18    Ir. 221                                                                             Si/KCl                                                                               100/9/1                                                                               N.sub.2,                                                                             1000° C.                                                                     darkblue/                                   (Hg 20)                 1.5 l/min. lilac                                       __________________________________________________________________________     Color meter values                                                                   45°/0°                                                                       22.5°/22.5°                                                                 Electro-                                          Example                                                                              white background                                                                           black background                                                                          conductivity                                      No.   L   a   b   L  a   b   tivity  Magnetivity                               __________________________________________________________________________     10    31.3                                                                               -2.9                                                                               -5.2                                                                               63.8                                                                              +0.9                                                                               -9.7                                                                               920 kΩcm                                                                         Little                                    (Hg 7)                                                                         11    49.8                                                                               -0.3                                                                               +6.0                                                                               72.4                                                                              -2.1                                                                               -2.8                                                                               80.8                                                                               MΩcm                                                                         Much                                      (Hg 10)                                                                        12    54.6                                                                               -0.6                                                                               +6.7                                                                               80.1                                                                              +1.8                                                                               -3.1                                                                               >628                                                                               MΩcm                                                                         Much                                      (PM 30-1)                                                                      13    59.4                                                                               -1.2                                                                               +4.9                                                                               84.6                                                                              +2.2                                                                               -4.9                                                                               <628                                                                               MΩcm                                                                         None                                      (PM 30-5)                                                                      14    39.2                                                                               -2.9                                                                               -2.5                                                                               76.2                                                                              +1.7                                                                               -10.9                                                                              2.0 MΩcm                                                                         None                                      (MK 24)                                                                        15    52.5                                                                               -2.6                                                                               2.2 23.8                                                                              +0.8                                                                               -17.0                                                                              60  MΩcm                                                                         None                                      (Be 523)                                                                       16    23.0                                                                               ±0.0                                                                            -7.6                                                                               34.5                                                                              +10.9                                                                              -20.0                                                                              176 kΩcm                                                                         Little                                    (Hg 29)                                                                        17    24.5                                                                               +0.2                                                                               -8.2                                                                               25.3                                                                              +18.5                                                                              -28.4                                                                              45  kΩcm                                                                         Little                                    (Hg 18)                                                                        18    25.2                                                                               ±0.0                                                                            -7.8                                                                               36.0                                                                              ±19.2                                                                           -30.4                                                                              212 kΩcm                                                                         Little                                    (Hg 20)                                                                        __________________________________________________________________________     Powder mixture          Calc. conditions                                                  Reduction                                                                            Mixing ratio Calcine                                          Example                                                                              TiO.sub.2                                                                           agent/                                                                               pig./red./                                                                            Gas   temp.                                                                               Color masstone/                             No.   mica pig.                                                                           halide                                                                               halide flow rate                                                                            (30 min.)                                                                           interference                                __________________________________________________________________________     19    Ir. 221                                                                             Si/CaCl.sub.2                                                                        100/9/1                                                                               N.sub.2,                                                                             1000° C.                                                                     black/                                      (Hg 17)                 1.5 l/min. darkgold                                    20    Ir. 123                                                                             Si/MgCl.sub.2                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     bluegrey/                                   (PM 20-1)               3.0 l/min. blue                                        21    Ir. 123                                                                             Si/FeCl.sub.2                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     bluegrey/                                   (PM 20-5)               3.0 l/min. blueish silver                              22    Ir. 123                                                                             Si/FeCl.sub.3                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     bluegrey/                                   (PM 20-6)               3.0 l/min. blueish silver                              23    Ir. 123                                                                             Si/NiCl.sub.2                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     darkblue-grey/                              (PM 22-5)               3.0 l/min. blue                                        24    Ir. 123                                                                             Si/CuCl.sub.2                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     grey/silver                                 (PM 22-6)               3.0 l/min.                                             25    Ir. 123                                                                             Si/MnCl.sub.2                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     grey/silver                                 (PM 22-7)               3.0 l/min.                                             26    Ir. 123                                                                             Si/CrCl.sub.3                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     blueish grey/                               (PM 22-8)               3.0 l/min. blueish silver                              __________________________________________________________________________     Color meter values                                                                   45°/0°                                                                       22.5°/22.5°                                                                 Electro-                                          Example                                                                              white background                                                                           black background                                                                          conductivity                                      No.   L   a   b   L  a   b   tivity  Magnetivity                               __________________________________________________________________________     19    11.7                                                                               -1.1                                                                               -0.1                                                                               32.4                                                                              +2.1                                                                               +1.7                                                                               28 kΩcm                                                                          Much                                      (Hg 17)                                                                        20    127.7                                                                              -2.6                                                                               -8.5                                                                               49.7                                                                              +1.9                                                                               -17.4                                                                              --      --                                        (PM 20-1)                                                                      21    26.8                                                                               -2.9                                                                               -8.1                                                                               59.6                                                                              +1.1                                                                               -17.0                                                                              --      --                                        (PM 20-5)                                                                      22    29.9                                                                               -3.2                                                                               -7.8                                                                               61.9                                                                              +1.0                                                                               -16.5                                                                              --      --                                        (PM 20-6)                                                                      23    18.4                                                                               -1.7                                                                               -7.7                                                                               44.9                                                                              +3.0                                                                               -16.9                                                                              --      --                                        (PM 22-5)                                                                      24    24.7                                                                               -3.0                                                                               -8.4                                                                               54.2                                                                              +1.5                                                                               -18.7                                                                              --      --                                        (PM 22-6)                                                                      25    27.9                                                                               -3.1                                                                               -8.6                                                                               60.0                                                                              +1.2                                                                               -18.2                                                                              --      --                                        (PM 22-7)                                                                      26    25.2                                                                               -2.9                                                                               -9.2                                                                               56.0                                                                              +1.4                                                                               -19.0                                                                              --      --                                        (PM 22-8)                                                                      __________________________________________________________________________     Powder mixture          Calc. conditions                                                  Reduction                                                                            Mixing ratio Calcine                                          Example                                                                              TiO.sub.2                                                                           agent/                                                                               pig./red./                                                                            Gas   temp.                                                                               Color masstone/                             No.   mica pig.                                                                           halide                                                                               halide flow rate                                                                            (30 min.)                                                                           interference                                __________________________________________________________________________     27    Ir. 123                                                                             Si/CeCl.sub.3                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     grey/                                       (PM 29-3)               3.0 l/min. blueish silver                              28    Ir. 123                                                                             Si/CoCl.sub.3                                                                        100/3/1.5                                                                             N.sub.2,                                                                              800° C.                                                                     darkblue-                                   (PM 29-6)               3.0 1/min. grey/blue                                   29    Ir. 123                                                                             Si/CaCl.sub.2                                                                        100/2/10                                                                              3.0 l/min.                                                                            800° C.                                                                     blueish dark-                               (PM 26-2)                          grey/bluesilver                             30    Ir. 123                                                                             Si/CaCl.sub.2                                                                        100/2/5                                                                               3.0 l/min.                                                                            800° C.                                                                     blueish dark-                               (PM 26-3)                          grey/bluesilver                             31    Ir. 123                                                                             Si/CaCl.sub.2                                                                        100/2/2                                                                               3.0 l/min.                                                                            800° C.                                                                     blueish grey/                               (PM 26-4)                          blueish silver                              32    Ir. 123                                                                             Si/CaCl.sub.2                                                                        100/3/0.5                                                                             3.0 l/min.                                                                            800° C.                                                                     lightgrey/                                  (PM 19-2)                          blueish silver                              __________________________________________________________________________     Color meter values                                                                   45°/0°                                                                       22.5°/22.5°                                                                 Electro-                                          Example                                                                              white background                                                                           black background                                                                          conductivity                                      No.   L   a   b   L  a   b   tivity  Magnetivity                               __________________________________________________________________________     27    27.1                                                                               -3.1                                                                               -8.6                                                                               59.1                                                                              +1.0                                                                               -16.3                                                                              --                                                (PM 29-3)                                                                      28    18.9                                                                               -1.8                                                                               -7.8                                                                               47.1                                                                              +2.6                                                                               -16.3                                                                              --      --                                        (PM 29-6)                                                                      29    26.0                                                                               -2.8                                                                               -9.8                                                                               54.6                                                                              +1.0                                                                               -17.4                                                                              --      --                                        (PM 26-2)                                                                      30    27.9                                                                               -3.2                                                                               -8.6                                                                               58.7                                                                              +0.8                                                                               -17.1                                                                              --      --                                        (PM 26-3)                                                                      31    31.2                                                                               -3.4                                                                               -7.6                                                                               62.8                                                                              +0.5                                                                               -14.9                                                                              --      --                                        (PM 26-4)                                                                      32    41.34                                                                              -2.95                                                                              -2.36                                                                              78.14                                                                             1.23                                                                               -8.97                                                                              --      --                                        (PM 19-2)                                                                      __________________________________________________________________________ 

We claim:
 1. A platelet-shaped colored pigment comprising a platelet-shaped substrate coated with a coating layer consisting of50-99% by weight of titanium oxides including titanium dioxide and at least one suboxide of titanium and 1-50% by weight of silica,such that the coating layer has a surface adjacent to the substrate and an outer surface, wherein the concentration of the titanium oxides in the coating layer is maximum at the surface adjacent to the substrate and gradually decreases towards the outer surface, and the concentration of silica is maximum at the outer surface and gradually decreases towards the surface adjacent the substrate.
 2. The pigment of claim 1, wherein the coating layer consists of70-98% by weight of the titanium oxides and 2-30% by weight of silica.
 3. The pigment of claim 1, wherein the platelet-shaped substrate is a mica, kaolin or glass platelet optionally coated with one or more metal oxide layers or a metal or metal oxide platelet coated with TiO₂.
 4. The pigment of claim 1, wherein the substrate has a diameter of 1 to 500 μm and thickness of 0.1 to 1 μm.
 5. A process for preparing a platelet-shaped colored pigment, which comprises mixing a platelet-shaped substrate coated with TiO₂ with Si powder as a solid reducing agent to form a mixture and heating the mixture in a non oxidizing atmosphere at a temperature of more than 600° C.
 6. The process of claim 5, wherein said substrate coated with TiO₂ and said Si powder are mixed in a weight ratio from 200:1 to 5:1.
 7. The process of claim 5, wherein said substrate coated with TiO₂ and said Si powder are mixed in a weight ratio from 100:1 to 10:1.
 8. The process of claim 5, wherein a halide is admixed with said platelet-shaped substrate coated with TiO₂ in an amount of from 0.1 to 40% by weight of the substrate coated with TiO₂.
 9. The process of claim 8, wherein said halide is admixed with said platelet-shaped substrate coated with TiO₂ in an amount of from 0.5 to 10% by weight of the substrate coated with TiO₂.
 10. The process of claim 8, wherein said halide is MgCl₂, CaCl₂ or CeCl₃.
 11. The process of claim 6, wherein a halide is admixed with said platelet-shaped substrate coated with TiO₂ in an amount of from 0.1 to 40% by weight of the substrate coated with TiO₂.
 12. The process of claim 11, wherein said halide is admixed to said platelet-shaped substrate coated with TiO₂ in an amount of from 0.5 to 10% by weight of the substrate coated with TiO₂.
 13. The process of claim 11, wherein said halide is MgCl₂, CaCl₂ or CeCl₃.
 14. The process of claim 5, wherein the non oxidizing atmosphere is a N₂ or Ar atmosphere.
 15. The process of claim 5, wherein the temperature is of from more than 600° C. to 1100° C.
 16. The process of claim 5, wherein the mixture is heated for 10 to 60 minutes. 